ISIL has imposed strict rule in Raqqa - home to more than 220,000 people before the Syrian conflict - and committed atrocities against the civilian population since its takeover of the city more than two years ago.

Syria's conflict started with mostly unarmed demonstrations against President Bashar al-Assad in March 2011.

It has since evolved into a full-on civil war that has killed at least 270,000 people, according to the observatory.